GnRH is a decameric peptide released from the hypothalamus. In the anterior pituitary gland, GnRH activates the GnRH receptor. Activation of the GnRH receptor triggers the release of follicle stimulating hormone (FSH) and leuteinizing hormone (LH). FSH and LH stimulate the biosynthesis and release of sex steroids in the gonads of both genders.
Typically, this is desirable, but certain sex hormone dependent pathological conditions exist where it would be beneficial to prevent activation of the GnRH receptor. For example, inhibition of the GnRH receptor can lead to a large drop in sex steroid production, which in turn can alleviate sex hormone dependent pathological conditions such as prostate cancer, endometriosis, uterine fibroids, uterine cancer, breast cancer, ovarian cancer, testicular cancer, or primary hirsutism. Moreover, there are other situations where it would be beneficial to prevent activation of the GnRH receptor, such as during some points of the in vitro fertilization process, such as to prevent LH surge.
All currently marketed GnRH therapeutics are peptides that exhibit receptor antagonism in one of two ways. The first is through GnRH receptor superagonism. The GnRH receptor, when stimulated in bursts, causes normal release of the gonadotropins, FSH and LH. Under constant stimulation, the receptor becomes desensitized and the overall effect is GnRH receptor inhibition. The superagonism process is somewhat undesirable, as inhibition via this process can take up to two weeks to arise in human patients. During this delay there is often an increase in disease symptoms due to the initial hormone stimulation phase. This phenomenon is referred to as flare.
The second method for receptor inhibition is through direct antagonism of the GnRH receptor with peptide antagonists. This causes an immediate drop in plasma LH levels. However, as mentioned above, current pharmaceuticals that cause blockade of the GnRH receptor are all peptides. As such they are not orally bioavailable and must be administered via parenteral means such as intravenous, subcutaneous or intramuscular injection. Thus, an orally effective GnRH antagonist would be of significant benefit.
Therefore, based upon the foregoing, it is clear that GnRH receptor antagonists are useful, and development of new GnRH receptor antagonists is highly desirable.